The present invention relates to air compression systems, in particular to such systems employing a screw compressor driven by a motor such as a diesel engine or an electric motor, which also drives other equipment, and which continues to drive such equipment as well as the screw compressor even during periods of low compressed air consumption.
Motor-driven screw compressors provide a source of compressed air that performs many useful functions. Screw compressor systems have gained acceptance and significant growth due to their robustness, compactness and reliability. Designed for long periods (normally over 100,000 hours) of continuous operation, they provide up to 98% online availability. Their low maintenance costs together with their high energy efficiency minimizes operating costs. The smooth running action of the rotors enables screw compressors to handle the most difficult gases, contaminants, or liquid slugs without vibration.
Among the many examples of machines which use screw compressors are drilling rigs wherein a drill bit of a drill string is rotated to drill a hole in the ground, i.e., in earth and/or rock. In order to flush the cuttings from the hole as it is being drilled, it is common to employ a screw compressor to produce pressurized air which is conducted downwardly through the drill string to the front face of the drill bit. The cuttings become entrained in the airflow and are brought to the surface as the air travels upwardly along the exterior of the drill string. The pressurized air also serves to cool the cutting elements of the drill bit.
In the case of so-called percussive tools, the pressurized air also functions to reciprocate an impact piston which applies percussive blows from a piston to a rotating drill bit to enhance the cutting action. The piston is disposed below the ground surface immediately above the drill bit (i.e., a so-called down-the-hole hammer).
In many compressed air applications it is common to drive the screw compressor by a motor (i.e., a fuel-driven engine or an electrically driven motor), which also drives other equipment, such as a hydraulic system which functions to: power hydraulic motors to raise and lower the drill string, add drill rods to the drill string as drilling progresses, remove drill rods from the drill string as the drill string is being withdrawn from the hole, raise and lower a drilling mast, raise and lower leveling jacks, and propel the drilling rig (in the case of a mobile drilling rig). The motor also drives a hydraulic pump and a cooling fan of a cooling system.
The compressed air needs of such a drilling machine are associated with the supplying of flushing air for flushing cuttings and/or driving the impact piston of a percussive tool. Thus, for long periods during operation of the drilling rig, there is no need for pressurized air, such as during the adding or removal of drill rods, relocating the drill rig, setting up the drill rig, lunch breaks etc. Although there is no need during those periods to circulate compressed air to flush cuttings or to reciprocate the impact piston, it is still necessary to drive the motor in order to power the hydraulics.
In a typical air compressing system, the drive connection between the screw compressor and the motor is such that the screw compressor is driven whenever the motor is driven, despite the fact that continuous operation of the screw compressor is not necessary when drilling is not taking place. In an effort to reduce the wasted energy consumption of the motor in such a case, the air inlet of the screw compressor is closed, but that results in a reduction of perhaps only 25% of the energy required to drive the screw compressor, because even with its inlet closed, the screw compressor is still compressing air at its outlet, i.e., air trapped between the compressor outlet and a compressed air reservoir to which the outlet is usually connected.
There are certain measures that could be taken to further reduce the unnecessary consumption of energy. For example, a clutch could be provided between the engine and the screw compressor to unload the compressor during periods of low air requirements, but that would add considerable cost to the equipment, and the clutch would rapidly wear in situations where the compressor has to be unloaded frequently. It is uneconomical and impractical to switch the compressor on and off at frequent intervals. In that regard, even during periods where a large quantity of compressed air is not needed, smaller quantities may still be needed, whereupon the screw compressor may have to cycle on and off to keep the air reservoir sufficiently pressurized.
Another possible energy-saving measure involves the provision of a variable speed gear drive for unloading the screw compressor, but such a drive is complicated and relatively expensive, as would be a two-speed gear drive with clutches. With a variable speed gear drive, the rpm on the compressor could be reduced for reduced energy consumption.
A relatively low-cost possible measure involves driving the screw compressor with a hydraulic motor that can be easily stopped or slowed during periods of low pressure requirements. However, such drives are relatively inefficient (80% maximum), so any energy savings realized during periods of low compressed air consumption would be lost during periods of high air compressed consumption.
Therefore, it would be desirable to provide an air compressing system employing a motor-driven screw compressor which, despite being driven by the motor during periods of low air compressed consumption, minimizes power consumption in a relatively inexpensive, yet simple and reliable way.